The present invention relates to a circuit for driving a piezoelectric stack.
In a dot matrix printer or an ink jet printer, the printing head is often driven by a piezoelectric actuator which uses a piezoelectric stack. Piezoelectric actuators heretofore proposed for such an application may be exemplified by one designed for an oscillating drive mechanism as disclosed in U.S. Pat. No. 3,614,486. The disclosed type of piezoelectric actuator is constructed such that a piezoelectric stack develops a dimensional strain therein when applied with a voltage and the strain is imparted to a wire, ink drop producing mechanism or like reciprocating mechanical object to print out desired data. The dimensional strain caused by the application of a DC excitation voltage to the piezoelectric stack is turned into oscillation whose amplitude varies in a sinusoidal waveform in terms of time. An ordinary printing head is so constructed as to print out a single dot at each time of drive and such has to be implemented by a piezoelectric stack drive circuit which is capable of causing one period of sinusoidal oscillation every time an excitation voltage is applied.
A prior art drive circuit elaborated for the above purpose comprises, as will be described later in detail, a piezoelectric stack, a power source for applying an excitation voltage to the stack, and a resistor for limiting a rush current in the event of driving the stack. The problem with such a drive circuit is that the drive circuit tends to be damaged because it is charged instantaneously. Another problem is that after the drive the charge stored in the piezoelectric stack needs be dissipated or discharged by short-circuiting, resulting in waste of power.
Meanwhile, U.S. Pat. No. 3,975,650 teaches a drive circuit in which an inductance is connected in series with a piezoelectric stack in order to suppress an instantaneous current that flows through the stack. This circuit, however, lacks the construction for dissipating the stored charge after the drive and, for this reason, it is inapplicable to a wire dot matrix printer head without noticeably lowering the quality of printed characters.